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An Interactive PZT Phase Diagram. To use, click on the
different regions of the diagram.
The cubic structure, found at high temperatures, does not
possess a spontaneous dipole.
The cubic structure means that there is no movement of
the Ti ion inside the O octahedra. This results in no net dipole moment,
making cubic PZT non-piezoelectric.
In the tetragonal phase, the most important atom motion
is that of the Ti atom in the O octahedra, in the originally  direction.
The atom forms a dipole moment in the  direction.
This gives it 6 possible directions to form, passing through each vertex
of the octahedra.
In the rhombohedral phase, the Ti atoms move in the 
direction. The unit cell has the same lattice parameter along each axis,
so there is a movement which is symmetrical about the  direction.
The movement of the Ti atom forms a dipole moment in the
 direction. This gives it 8 possible dipole moment directions,
through the centre of each octahedral face.